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    <title>Document</title>
    
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   <div>
       <p>圆的半径：<input id="r" type="text"></p>
       <p>圆的周长: <input id="cir" type="text"></p>
       <p>圆的面积：<input id="area" type="text"></p>
   </div>
   <script>
        var r=prompt('请输入半径');
        r=parseFloat(r)&&Number(r);
        if(!isNaN(r)){
            var cir=2*Math.PI*r;
            var area=Math.PI*r*r;
            document.getElementById('r').value=r;
            document.getElementById('cir').value=cir.toFixed(2);
            document.getElementById('area').value=area.toFixed(2);
        }
        else{
            alert('请输入正确的数字');
        }
        </script>
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